php用gd库生成图片后上载有关问题

//创建一个真彩画布<br />
$image = imagecreatetruecolor(400,190);<br />
//背景创建颜色<br />
$green = imagecolorallocate($image,255,255,255);<br />
//填充画布颜色<br />
imagefill($image,0,0,$green);<br />
//输出图片<br />
header("Content-Type: image/jpeg");<br />
imagejpeg($image);<br />
//销毁资源<br />
imagedestroy($image);

<a href="imagedown.php?filename=image.php"><img  src="image.php" / alt=" php用gd库生成图片后上载有关问题 " ></a>

<br />
if(isset($_GET['filename'])){<br />
????????//var_dump(getimagesize($_GET['filename']));<br />
????????download($_GET['filename']);<br />
????}<br />
????function download($fileName){<br />
????????header("Content-Type: image/jpeg");<br />
????????header('Content-Disposition: attachment; filename="'.$fileName.'"');<br />
????????header('Content-Length: '.filesize($fileName));<br />
????????readfile($fileName);<br />
????}<br />

download('x.jpg');<br />
<br />
function image() {<br />
  //创建一个真彩画布<br />
  $image = imagecreatetruecolor(400,190);<br />
  //背景创建颜色<br />
  $green = imagecolorallocate($image,255,255,0);<br />
  //填充画布颜色<br />
  imagefill($image,0,0,$green);<br />
  //输出图片<br />
  //header("Content-Type: image/jpeg");<br />
  imagejpeg($image);<br />
  //销毁资源<br />
  imagedestroy($image);<br />
}<br />
<br />
function download($fileName){<br />
  ob_start();<br />
  image();<br />
  $s = ob_get_clean();<br />
  header("Content-Type: image/jpeg");<br />
  header('Content-Disposition: attachment; filename="'.$fileName.'"');<br />
  header('Content-Length: '.strlen($s));<br />
  echo $s;<br />
}